1. Field of the Invention
This invention relates to a toner for developing electrostatic images, and a process for producing it.
2. Related Background Art
To conduct electrophotography, a large number of methods are known in the art as disclosed, for example, in U.S. Pat. No. 2,297,691. In general, in such electrophotography, an electrostatic latent image is formed on a photosensitive member, utilizing a photoconductive material and according to various means, and subsequently the latent image is developed using a toner to form a toner image. The toner image is transferred to a transfer medium such as paper if necessary, and then the toner image thus transferred is fixed to the transfer medium by heating, pressing, heat-pressing or using solvent vapor. A copy is thus obtained. At present, fixing methods that utilize heat are in general use because copied images can be firmly fixed, transferred images can be readily handled and fixing can be comfortably operated. As heat-fixing methods, there are methods that utilize radiation heat as in the heat chamber method. In view of a high thermal efficiency, a high-speed response and a high safety, a heat-roll fixing method in which a heated roll-like heating member (e.g., a heat roll) is brought into pressure contact with toner images to fix them is most widely employed in copying machines. Although this method has a high thermal efficiency, the energy used for heat melting occupies a rather large position in copying machines. Also, molten toner images come in direct contact with the heat roll, so that the toner may adhere to the heat roll to cause stains on subsequent images, i.e., an offset phenomenon, or, in an extreme instance, to make the transfer medium wind around the heat roll, i.e., a wind-around phenomenon.
As a process for producing toners, it is known to use a process in which a colorant such as a dye or pigment and additives such as a charge control agent are melt-kneaded into a thermoplastic resin (binder resin) to carry out uniform dispersion, thereafter the resulting melt-kneaded product is cooled, the cooled product is pulverized, and the pulverized product is classified to produce toner particles having the desired particle diameters.
In the process for producing toners by such pulverization, there are limitations when a release agent such as wax is added to toner particles. That is, in order to disperse the release agent at a satisfactory level, the release agent must not dissolve to turn into liquid at the temperature where it is kneaded with the binder resin, and the release agent must be used in a large content. Because of such limitations, it is not easy to improve fixing performance of toners.
Meanwhile, it is proposed to use a method in which a toner containing a release agent such as wax is produced by suspension polymerization or emulsion polymerization. In this suspension polymerization, a monomer composition is prepared by uniformly dissolving or dispersing a colorant (optionally together with a polymerization initiator, a cross-linking agent, a charge control agent and other additives) in polymerizable monomers, and thereafter the monomer composition is dispersed by means of a suitable stirrer in a continuous phase (e,g, an aqueous phase) containing a dispersion stabilizer, to carry out polymerization to obtain toner particles having the desired particle diameters.
In this suspension polymerization, the monomer composition is granulated in a highly polar dispersion medium such as water, and hence the particles formed take a pseudo-capsular structure in which components having polar groups, contained in the monomer composition, are present in the surface layer portion which forms the interface between the particles and the dispersion medium, and non-polar components are present in the inside. Utilizing such a characteristic of this method, it is possible to encapsulate a low-melting wax in the toner particles.
Because of the encapsulation of a low-melting wax, the toner produced by polymerization can achieve both anti-blocking and low-temperature fixing, which are performances conflicting with each other. Since the low-melting wax is encapsulated in toner particles, the wax melting at a lower temperature contributes to an improvement in thermal conductivity of toner without lowering anti-blocking properties, so that it becomes possible to fix the toner at a lower temperature. As an additional advantage, the wax having melted at the time of fixing also acts as a release agent, and hence it is possible to prevent high-temperature offset without applying a release agent such as silicone oil to the fixing roll.
When the low-melting wax is, however, dissolved or dispersed in a large quantity in the monomer composition to carry out granulation in an aqueous medium, it is difficult to carry out the granulation compared with the case of a monomer composition containing no wax, tending to allow the resulting toner to have a broad particle size distribution.